I've been looking into Cozy for rent collection and potential late fee reminder but wonder if its worth it- or just have them use Venmo, Zelle or Paypal instead...

First, because of your musical background, you're already light years ahead of many experienced investors. 

The RR & Collection report, CapEx report, and past years' P&L's, including T-12 should be all you need from the owner to make an offer.

I am a new landlord and looking to find the best method of collecting rent?

Would love some feedback and this one is tricky with the 7-day because most of the time if I put a 7-day which is rare... it's for them having a dog, parking over the water meter, loud music, too many people in the place, etc.   

I plan to collect first and security at lease signing and give the keys around date of possession (after proof of renters insurance, utilities check, final walk through etc.) but I don't know if this is the best option.If I collect first and security at lease signing (in a few days) and they don't move in for any reason.

For me it would make sense that you don't necessarily need to sell, you could ideally refi, have the same amount of money leftover, and do what you wanted with that money anyways, and continue to collect cash flow from those first two properties as well as any other ventures that you use the cash for.

I would give them every opportunity to collect them, but once proper notice has been provided and you need to move forward, I would clear it with an attorney in your area, but it seems like it would be considered yours at this point.

Assumptions in general (case by case, but this is typical)- Rent $ amounts collected from Rentometer - amount based on interior finishes, potential amenities- Vacancy at 5-8%, repairs 5%, 10% cap ex, 10% property management  - are these usually higher in C class areas?

., collecting rents and handling evictions).